Crankshaft: I have been through, trying to figure it out mathematically. And believe there are too many variables to make a mathematical equation truly accurate however, I would have to look it up or maybe: Colton will check in I think he has a pretty good explanation of it. However with differing light conditions differing eyesight for shooter styles of front sights and how the light refracts around them it will only get you close. The way I have done in the past you're going to need to find a friend preferably a fellow shooter who trust you very much, this being that you will need to have your rifle unloaded a block placed in a mechanism so it cannot possibly be fired as your friend who trust you very much, will be downrange with craft supplies paper scissors pushpins and staple gun. Let's say you were going to use a white sighter on a black background. I typically would put an oversized white sighter on a black background than have my friend take two pieces of black paper and progressively move them in giving you time to site at each one to find out when you have just a ghost line of white on each side of your site. As I've been running off at the mouth something else struck me, that Paul and I once tried along with a group of other shooters was once you find the proper with for your white square take a bright colored safe orange or bright red stripe and put immediately below it how far below may depend on your eyesight this is put there as you raise the rifle into the bottom of the site are you will see the colored stripe which I believe we were making about 1/2 to 3/4 of an inch wide as a warning bar that you're coming into the bottom your sighter and helps keep you from burying the front sight too far up into it.

Spotter size at 60 yards for a .055" wide front sight "should" be a simple geometry relationship.

Take one half of the front sight width (.0275") and divide it by the distance from your eye to the front sight(let's say 42") multiply that by the distance to the target in inches(60 yds. x 3 ft/yds x 12"/ft) and that number should be 1/2 of the width of the spotter you need. For this set of numbers it calculates out to a spotter that is 2.828" wide.

It should be a good place to start but it will need to be adjusted some I'm sure as we all see a little differently.

Robin's idea is probably the simplest and the most accurate if you can convince your buddies the gun's not loaded!!!

Last edited by cking (6/04/2016 7:59 AM)

If its a Lyman 17A with the inserts it should be simple otherwise if it won't come out perhaps a set of dividers could be slid down in the globe and used to get the width?

Been thinking about this some lately while making spotters, and I would like to submit a further simplification of Carl's approach.  Carl appropriately set up the calculation as a proportion, but used radius for bead and spotter size; I simply replaced radius with diameter, so that you don't have to divide bead size by 2 or multiply result by 2.

(bead_diameter / distance_from_bead_to_eye) * distance_to_target_in_inches = spotter_diameter

So, for the original question, for a chunk spotter at 60 yards with a .055" bead 42" from the shooters eye:
(.055/42) * 2160 = 2.83"  #same as calculated by Carl's method

For a table match spotter at 30 yards:
(.055/42) * 1080 = 1.41"

Also, for convenience, I'll throw in the values for distance to target at common ranges in inches:
60 yards = 2160"
30 yards = 1080"
50 yards = 1800"
25 yards =  900"

Again, this is just offered as an alternate, simplified method that may be useful to somebody.

Last edited by barton (7/22/2016 4:19 PM)